Do not assume that if you pay more you will get a better instructor, that is not the case in the driver training industry, national driving schools will charge more because of their overhead costs not because they have better instructors, many national driving schools do use trainee instructors so you have been warned.
Check out their website see if they quote pass rates, ask if they use trainee instructors, ask if they supply training records they should, this should record what you have covered and at what level you have achieved.
A driving instructor needs your training record so they can update it on a regularly basis so they know where you are in your training and at what level it is important to have one.
So the things to ask are will I get a fully qualified Instructor, what are your pass rates,will I get a training record, what is your hourly rate, please remember do not assume that if you are paying more you will get a better instructor it relates to overhead costs the bigger the company the greater the overhead costs simple as that, even if some one recommends a driving school do find out the other information first, most driving schools do offer a free lesson or cash payments to a pupil if they get a referral from them, so do not just base your choice on that fact only. The best referral is from some who has previously passed so has nothing to gain from it.